The delicate, spiderweb-like (therefore the name) arachnoid layer, attached to the inside of the dura, surrounds the brain and spinal cord but does not line the brain down into its sulci (folds). Cerebrospinal fluid flows under this membrane in the subarachnoid space, which is full of the delicate fibres of the arachnoid extending down to attach to the pia mater.

The portions covering the brain and spinal cord are called arachnoidea encephali and arachnoidea spinalis, respectively.

The arachnoid and pia mater are sometimes considered as a single structure, the leptomeninx, or the plural version, leptomeninges. (Lepto- from the root meaning thin in Greek). Similarly, the dura in this situation is called the pachymeninx.

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Etymology

Arachnoid is from a Greek root, and means cob web like. The mater designation (meaning mother in Latin) is borrowed from the dura mater and pia mater, which were Latin translations of Arabic terms. While mater does not technically belong with the arachnoid layer, it has nevertheless been adopted by it for uniformity with the other meninges, and arachnoid mater is currently the Terminologia Anatomica international standard.